Vertical-cavity surface-emitting lasers (VCSELs) have many advantages over traditional edge-emitting lasers, such as low cost manufacturing, high yield, good beam quality, and scalable geometries. These properties make VCSELs desirable for many applications. For example, K. H. Hahn, M. R. T. Tan, and S. Y. Wang describe using VCSELs in multimode fiber data links in Modal and Intensity Noise of Large-Area Multiple-Transverse-Mode VCSELs in Multimode-Optical-Fiber Links, 1994 CONFERENCE ON OPTICAL FIBER COMMUNICATION--paper ThB3, OFC '94. However, the turn-on jitter (variation in turn-on delay time) of VCSELs can limit the maximum bit rate that can be achieved in such data links. This problem is described by C. R. Mirasso, P. Colet, and M. San Miguel in Dependence of Timing Jitter on Bias Levelfor Single-mode Semiconductor Lasers under High Speed Operation, QE-29 IEEE J. QUANTUM ELECTRON., pp. 23-32, (1993), by A. Sapia, P. Spano, C. R. Mirasso, P. Colet, and M. San Miguel in Pattern Effects in Timing Jitter of Semiconductor Lasers, 61 APPL. PHYS. LETT., pp. 1748-1750 (1992), and by T. M. Shen in Timing Jitter in Semiconductor Lasers under Pseudorandom Word Modulation, 7 J. LIGHTWAVE TECHNOLOGY, pp. 1394-1399, (1989).
It is known that the turn-on delay of a VCSEL varies depending on the off time of the VCSEL, i.e., the time that the laser current was switched off before it switched on again. Since the off time of the VCSEL varies according to length of the run of 0s in the data modulating the laser current, the turn-on delay of the VCSEL is subject to a data-dependent jitter. FIGS. 1A and 1B illustrate the bit pattern-dependence of the turn-on delay time of a single-mode VCSEL. FIG. 1A shows the time dependence of the photon density during the turn-on process with two extreme bit patterns. FIG. 1B shows the time dependence of the photon density during the turn-on process with the two extreme bit patterns and with some intermediate bit patterns. In FIGS. 1A and 1B, the bit duration is 1 ns, and the laser current is switched between an OFF state of about one-half of the threshold current of the VCSEL and an ON state of about five times the threshold current. In FIG. 1A, the turn-on delay time illustrated by curve A, in which a long string of 0s precedes switching to 1 is considerably longer than the turn-on delay time illustrated by curve B, in which a long string of 1s is followed by a single 0 bit before switching back to 1. Comparing curve A with curve B shows that the turn-on delay time varies depending on the bit pattern of the laser current.
FIG. 1B shows the variation of the photon density in response to several different pseudo-random word patterns that include the extreme bit patterns shown in FIG. 1A with some intermediate bit patterns. Each trace represents a run of a different number of 0s between consecutive 1s. The difference between the maximum and minimum turn-on delay is called the jitter spread. The jitter spread imposes a limitation on the maximum rate at which the VCSEL can be modulated.
In addition to being subject to jitter spread, VCSELs are also subject to spatial hole burning. In VCSELs, light is generated by the laser current flowing through a large cross-sectional area of semiconductor material in the quantum well region. In some applications, a VCSEL is required to emit a light beam having a single Gaussian intensity distribution. This is known as single mode operation, and the light beam generated by the laser in single mode operation will be called a single light beam. VCSELs emit a single light beam when the laser current is just above the threshold level. However, when the laser current is increased beyond a second, higher, threshold level, the laser begins to emit a light beam with a double, or higher, Gaussian intensity distribution.
The VCSEL generates the single light beam in a central stimulated emission zone of the quantum well region. Since light generation depletes the density of carriers depending on the intensity of the light generated, generation of the single light beam results in a depletion zone forming in the center of the stimulated emission zone, and a corresponding increase in the carrier density in a zone surrounding the depletion zone. Spatial hole burning occurs when the carrier density in the depletion zone falls below the threshold, so that light is no longer generated in the depletion zone. The light beam generated by the VCSEL then assumes a double Gaussian intensity profile. At higher laser currents, additional depletion zones may form in the stimulated emission zone, resulting in the light beam generated by the VCSEL having a multiple Gaussian intensity profile. Such an intensity profile makes the VCSEL unsuitable for use in applications in which a light beam having a single Gaussian intensity distribution is required.
The limiting effect of spatial hole burning on the maximum intensity at which VCSELs can generate a single light beam makes VCSELs unsuitable for certain applications. For example, a laser suitable for writing on a magneto-optical disc is required to generate a single light beam with a power of about 30 mW, whereas the highest single light beam power that can be generated by known VCSELs is in the range of 1-2 mW.
Accordingly, to increase the bit transmission rate of VCSEL-based optical communication systems, and to increase the speed at which the light output of the VCSEL of a laser printer can be modulated, and for other applications in which the light output of the VCSEL must be modulated at high speed, a VCSEL having reduced jitter spread is required. Also, to enable VCSELs to be used in applications in which a single light beam having a high intensity is required, a VCSEL is needed in which the onset of spatial hole burning occurs, if at all, at a substantially-increased light intensity compared with known VCSELs.